1. Field of the Invention
The present invention relates to a device for the optical abutment of photosensitive detectors disposed in the form of strips.
In some fields, such for example as taking pictures of the earth by satellite, alignments are required of a large number of photosensitive detectors, for example 10,000 detectors, the detectors must be positioned very accurately, with a pitch of a few tens of micrometers for example. To form these lines of detectors, strips are provided comprising a limited number of detectors which is about 2000 in the case of silicon detectors and a few hundreds only in the case of detectors formed on ternary or quaternary semiconductors, having as formula for example Hg Cd Te or Ga In As P.
2. Description of the Prior Art
For forming these lines of detectors it is known to place a sufficient number of strips end to end.
In the prior art this abutment is provided optically or mechanically.
Mechanical abutment consists of bonding strips to a substrate while respecting the pitch of the detectors as is shown in FIG. 1.
In FIG. 1, two strips 1 and 2 are placed end to end. They carry detectors referenced d.
Mechanical abutment has in particular the following disadvantages:
it is difficult to obtain for it requires accurate positioning of the strips on their support, in so far as their alignment is concerned and respecting the pitch between detectors of two adjacent strips;
at the junction of two strips, it frequently happens that at least one detector is missing and that the detectors adjacent the edges are damaged;
finally, when a strip carries a detector which is not operating, the signal to noise ratio is zero for this detector.
Optical abutment, such as it is known in the prior art, requires the use of an optical system formed of two identical prisms joined side by side by a semireflecting face.
This system is derived from the well known "Lummer's cube" which is used for separating a convergent beam into two beams, without introducing astigmatism. In the case of optical abutment, the optical system used is not a cube but a parallelepiped, for it must be possible to align the strips of the detectors on two of its faces.
FIG. 3 shows a side view of this optical system with the two prisms 4 and 5 joined together by their semireflecting face 6. The strips of detectors 1 and 3 are disposed on two right angle faces each belonging to a different prism. They are carried by a support 7.
In FIG. 2 this system is shown in a front view. In this view, the two faces of the prisms are shown carrying the strips referenced 1, 2, 3.
As is shown in FIG. 3, the optical system splits up the image to be analyzed, through its semireflecting face 6.
The strips carried by one of the faces of the system receive the image after reflection from the semireflecting face 6 and the strips carried by the other faces of the system receive the same image transmitted by the semireflecting face 6.
FIG. 2 shows how the strips 1, 2, 3 . . . are disposed.
One of the faces carries the strips of even rank and the other those of uneven rank. The strips are disposed symmetrically with respect to the semireflecting face 6.
As is illustrated in FIG. 2, between two strips disposed on the same face, a gap is left corresponding substantially to a strip disposed on the other face. Thus, in FIG. 2, between strips 1 and 2 a gap is left corresponding to the strip 3 carried by the other face. In the example chosen, strips 1 and 2 receive the image by reflection and strip 3 receives the image by transmission. The abutment of the strips is thus provided optically.
The known optical abutment of the prior art has the essential drawback of having a low signal to noise ratio because of the semireflecting layer.
The loss of brightness caused by this layer is at least equal to a factor 2. In fact, this loss is often much higher and equal to a factor of 3 to 4.
In fact, it is generally desired to provide radiometric functions, i.e. to know accurately the spectral reflectivity characteristics of the received image. The detectors then operate in a fairly wide spectral range and with variable incident polarization.
For the reflection and transmission coefficients to be equal, even with variable incident polarization, a metal semireflecting layer must be used having a nickel or chromium basis for example. The optical balance of these layers is for example as follows: 30% of the signal is transmitted, 30% of the signal is reflected and 40% of the signal is absorbed.
With respect to mechanical abutment in which it is assumed that the signal to noise ratio is equal to 1, the signal to noise ratio in the case of optical abutment, for the same detector technology, is only about 0.3.
As for the case of mechanical abutment, in the prior art optical abutment the presence of a defective detector in a strip results in a zero signal to noise ratio.
With respect to mechanical abutment, the optical abutment of the prior art generally causes no disturbance to the junction of the strips. There are generally no missing points at the junction of the strips, and the points adjacent the edges of the strips are not damaged.
The present invention relates to a device for the optical abutment of photosensitive detectors which, while keeping the essential advantages of the known prior art optical abutment device has additional advantages in that the signal to noise ratio is which is much higher going from 0.3 to about 0.7. And signal to noise ratio equal to about 0.5 is obtained in the case of an inoperative detector. Thus an accidental dead point in a strip does not result in a blind point in the reconstituted line of detectors, but only in a point having a lower signal to noise ratio, provided that the presence of this inoperative detector has been detected. This correction is particularly interesting in the case of infrared detectors which use semiconductors where isolated defects are more frequent than in silicon, and for which the efficiency in manufacturing these strips is very low.
Moreover, the optical abutment device of the invention has a redundancy of detectors which is a great asset in space applications.